The basic measuring component of a gas sensor is an electrochemical cell, which includes at least two electrodes in contact with one another via an electrolyte (that is, an ionic conductor). On the side of the cell which is open to the atmosphere, gas can flow to one of the electrodes (the working or sensing electrode), at which it is electrochemically converted. The current generated by the conversion is proportional to the quantity of gas present. A signal, which can, for example, be used to provide an alarm, is generated from the current. A variety of electrolyte systems are described in the literature. Sulfuric acid is one of the most commonly used electrolytes, and is used in sensors for common gases, such as, for example, CO, H2S, or O2. See, for example, U.S. Pat. No. 3,328,277.
As some analyte gases are sufficiently reactive only in neutral electrochemical media, aqueous electrolytes including a neutral or a basic inorganic salt as a conducting salt have also been described. See, for example, U.S. Pat. No. 4,474,648 and German Patent No DE 4238337.
The electrolytes described therein are hygroscopic (that is, they can absorb water from the surround environment). A hydroscopic electrolyte can be desirable for use in dry or low-humidity environments to delay drying of the cell. In high-humidity environments, however, a hydroscopic electrolyte can absorb so much water that electrolyte leaks from the cell. To prevent this leakage of electrolyte, a sensor cell typically includes an extra or reserve volume of approximately five times to seven times its electrolyte filling volume. Inclusion of such a substantial reserve volume cuts against a general aim of reducing the overall size of sensor cells.
In a number of sensors, organic liquids, which include conducting salts admixed therein to ensure ionic conductivity, are used as electrolytes to limit water absorption in high-humidity environments. See, for example, U.S. Pat. No. 4,169,779. The advantage at high relative humidity, however, becomes a disadvantage at low humidity and/or high ambient temperatures, as vaporized solvent cannot be reabsorbed from the atmosphere and is thus irrecoverably lost from the sensor cell.
Ionic liquids (IL) have also been used as electrolytes. Ionic liquids are defined as liquid salts with a melting point below 100° C. The salt-like structure of certain ionic liquids results in the absence of a measurable vapor pressure. The properties of ionic liquids vary substantially and are dependent, for example, upon the type and the number of organic side chains present in the ionic liquid, as well as the anions and cations thereof. Ionic liquids are available having melting points below −40° C. Many ionic liquids are both chemically and electrochemically stable and exhibit high ionic conductivity. A number of ionic liquids are not measurably hygroscopic. Such properties make ionic liquids good electrolytes in electrochemical gas sensors.
The use of ionic liquids in gas sensors was first described for use in connection with high sulfur dioxide concentrations. Cai et al., Journal of East China Normal University (Natural Science), article number 1000-5641(2001)03-0057-04. The use of ionic liquids as electrolytes in gas sensors is also disclosed, for example, in Great Britain Patent No. GB 2395564, U.S. Pat. No. 7,060,169 and published German patent application DE 102005020719. GB 2395564 describes the use of ionic liquids as electrolytes generally. U.S. Pat. No. 7,060,169 discloses the use of pure imidazolium and pyridinium salts as ionic liquid electrolytes. Published German patent application DE 102005020719 discloses the possibility of forming an open gas sensor without a diffusion membrane. The potential for the use of such technology in miniaturizing sensors is described in published German patent application DE 102004037312.
Although ionic liquids are used in a number of gas sensors as a replacement for classic (aqueous) electrolytes, the chemical processes in ionic liquids differ fundamentally from those in aqueous and organic systems, and the chemical processes in ionic liquids are not well characterized. See, for example, P. Wasserscheid, Angew. Chem. 2000, 112, 3926-3945 and K. R. Seddon, Pure Appl. Chem. Vol. 72, No. 7, pp. 1391-1398, 2000.